alternatehistory.com

In her youth, Blanche of Castile visited the Abbey of Santa María la Real de Las Huelgas, founded by her parents, she would at times would be sent letters by her cousin Arthur I of Brittany who wanted to marry her until his untimely death on 1202 in Mirabeau, on the death of her sister, Mafalda who was betrothed to Ferdinand of Leon, she would be the one chosen to replace her on the marriage which would happen on 1205 after the death of her sister.

Blanche of Castile would marry the heir to the King of Leon and the stepson of her sister, Berenguela, the marriage would produce two children, Alfonso X of Leon(1206) and Fernando(1208) which would be left under the care of Alfonso IX of Leon, Alfonso X of Leon would reign as King on the death of Alfonso IX and a bride was easily obtained for King Alfonso X on the 1220s by his grandfather and it is Isabel of Swabia, a daughter of the late Philip I of Germany who would provide his grandson of political connections and he was not wrong regarding that.

On the 1216, Berenguela, her sister would convince her sister to remarry to the heir of Navarre, the Count of Champagne, a dashing young knight, Theobald, the second marriage would produce three children; Blanche b. 1218, wife of John I of Brittany, Theobald II of Navarre b. 1220 and Henry b. 1222.

@Kurt_Steiner @isabella

Note:
Blanche of Castile is the mother of the King of Leon and King of Navarre here.